In recent years, due to sharply increased requirements for the mobile voice service, Global System for Mobile Communication(GSM) networks have been developed considerably. However, it is increasingly difficult for limited frequency resources to meet people's communication requirements, particularly in population-intensive urban areas. Along with an increase in an aging degree of GSM network devices, there is an urgent need to perform capacity expansion for the existing GSM networks. On the other hand, due to increasingly decreased expense for the voice service, an operator needs to effectively reuse existing hardware resources and frequency resources. Therefore, how to improve system capacity without increasing the existing system frequency resources has become a very important research project.
Multiple User Reusing One Slot(MUROS) techniques are mainly applied in 3rd Generation Partnership Project(3GPP) GSM/EDGE Radio Access Networks(GERAN), discussion for the Study Item stage, which seeks to improve user capacity of a GSM system and help the operator to eliminate a network burden, is started from the GERAN #36 conference in November 2007. The Work Item stage begins from the GERAN #40 conference in November 2008, and is formally named Voice services over Adaptive Multi-user channels on One Slot(VAMOS).
A VAMOS system needs to further improve voice capacity while reusing the existing network devices and wireless resources. In feasibility research phase, a candidate VAMOS solution primarily refers to multiplexing two or more users in the same slot without decreasing communication quality. At present, it is mainly considered to double the voice capacity of the system, i.e. two users reuse one slot. On one hand, this causes an influence on full-rate and half-rate voice channels(TCH/FS, TCH/HS, TCH/EFS, TCH/AFS and TCH/WFS), as well as corresponding associated control channels such as the Slow Associated Control CHannel(SACCH) and the Fast Associated Control CHannel(FACCH). On the other hand, after the VAMOS techniques are used, co-channel interference and adjacent channel interference increasingly occur as the number of the users in a cell increase, which results in a Carrier Interference Ratio(C/I) and frequency multiplexing decrease. There in a need to further investigate how to achieve a compromise between the frequency multiplexing decrease and the slot reusing improvement.
The existing VAMOS candidate solutions are mainly composed of three types as follows.
1) A Co-Traffic CHannel(Co-TCH) Scheme
Downlink: Gaussian Minimum Shift Keying(GMSK) baseband modulated signals in two channels (a difference between phases of the two channels is π/2) are linearly combined, and are transmitted after being radio modulated and power amplified.
Uplink: Each mobile station employs the GMSK modulation respectively and uses different Train Sequence Codes(TSC), and the multiplexed user signals in the two channels are separated out using a method such as joint detection at a base station side.
2) An Orthogonal Sub-Channel(OSC) Scheme
Downlink: The user signals in the two channels are transmitted using Quadrature Phase Shift Keying(QPSK), and the signal of every channel may be received by GMSK modulation at the user reception side.
Uplink: Each mobile station employs the GMSK modulation respectively and uses different Train Sequence Codes, and the multiplexed signals in the two channels are separated out using a method such as interference cancelation at a base station side.
3) An Adaptive Symbol Constellation mapping(ASC) Scheme
Downlink: With the alpha-QPSK scheme, transmission powers of an In-phase(I) channel and a Quadrature(Q) channel may be controlled by adaptively adjusting the constellation mapping.
Uplink: Each mobile station employs the GMSK modulation respectively and uses different Train Sequence Codes, and the multiplexed signals in the two channels are demodulated by a Multi-User Multiple-Input and Multiple-Output(MU-MIMO) receiver at a base station side.
At present, it is mainly considered to adopt the above 3 types of schemes in the VAMOS Work Item stage, which results in that two users may share the same time-frequency resource, so as to achieve the object of improving the system capacity by 2 multiples. However, which scheme is specifically used for the 3GPP GERAN is still in discussion.
Any of the schemes may be regarded as sharing two sub-channels of the same time-frequency resource, i.e. sub-channel 1 and sub-channel 2. Generally, sub-channel 1 is compatible with legacy mobile terminals and only VAMOS mobile phones can use sub-channel 2.
FIG. 1 is a structural diagram illustrating a frame of a GSM system. The GSM system employs Time Division Multiple Address(TDMA), where each TDMA frame is divided into 8 slots numbered as 0, 1, . . . , 7. The slot is a basic wireless resource unit for the GSM system, and each slot is a basic physical channel. A message format in a slot over the TDMA channel is called a Burst, i.e. each Burst is sent in a slot contained in the TDMA frame. According to a certain slot allocation principle, every mobile station is made to send a signal to the base station only in specified slots within each frame, and locations of such slots in the TDMA frame are fixed from one frame to another frame. In the case that timing and synchronization conditions are satisfied, the base station may receive the signal from each mobile station in each slot without interference thereamong. Also, the signals sent from the base station to multiple mobile stations are arranged sequentially and are transmitted in predetermined slots. Each mobile station can differentiate the signal sent to it from the combined signals if performs reception in the specified slot.
FIG. 2 is a structural diagram illustrating a multi-frame of the GSM system. In the GSM system, a TDMA frame has a frame length of 4.615 ms and contains 8 basic slots, thus each slot contains 156.25 symbols which occupy 15/26=0.577 ms. Multiple TDMA frames constitute the multi-frame, which has two structures composed of 26 or 51 TDMA frames respectively. When different logic channels are multiplexed into a physical channel, it is required to use such multi-frames. The multi-frame containing 26 TDMA frames are referred to as 26-multiframe, which has a time length of 120 ms and is used in the TCH and the associated control channel thereof, specifically as shown in FIG. 2. The TCH service channel may be divided into two types of full rate and half rate(the full-rate TCH and the half-rate TCH are usually represented as TCH/F and TCH/H), and the full-rate service channel has a transmission rate 2 times of that of the corresponding half-rate service channel. Using half of the available slots for the full-rate channel may obtain the half-rate channel. In terms of a quantity of the occupied burst, time for a full-rate channel may be used to transmit two half-rate channels, where these two half-rate channels are time division multiplexed. Thus, a carrier frequency may provide 8 full-rate or 16 half-rate service channels. In FIG. 2, the structural diagram illustrating the full-rate channel and the half-rate channel is provided. Specifically, A, a are corresponding SACCHs, I is an idle frame, and T, t are corresponding TCHs. As can be seen, for the full-rate TCH, the 0th˜24th frames are occupied within a 26-multiframe and the 25th frame is the idle frame. For the half-rate TCH, a 26-multiframe may transmit two half-rate TCHs, which are called a half-rate sub-channel 0 and a half-rate sub-channel 1 in the GSM standard. Specifically, the half-rate sub-channel 0 occupies 12 frames i.e. the 0th, 2nd, 4th, 6th, 8th, 10th, 13th, 15th, 17th, 19th, 21st, 23rd frames to send a voice block of the TCH/H channel, and sends an SACCH block over the 12th frame. The half-rate sub-channel 1 occupies 12 frames i.e. the 1st, 3rd, 5th, 7th, 9th, 11th, 14th, 16th, 18th, 20th, 22nd, 24th frames to send the voice block of the TCH/H channel, and sends the SACCH block over the 25th frame.
FIG. 3 is a schematic diagram illustrating downlink and uplink modulation for a VAMOS user group. In the downlink, bits from the TCH and the corresponding associated control channel of each user in the VAMOS user group are mapped onto an Adaptive Quadrature Phase Shift Keying(AQPSK) modulation symbol. At a reception side, the user demodulates the bit belonging to his/her TCH and corresponding associated control channel, and meanwhile performs wireless link measurement.
In FIG. 3, two coupled users employ the GMSK modulation and perform sending over the same time-frequency resource at the uplink in the same cell, i.e. the two users have the same slot number, Absolute Radio Frequency Channel Number(ARFCN) and TDMA frame number. Differentiation between the users depends on the Train Sequence Code(TSC) in data sent from the users. In the same cell, the user over the sub-channel 1 uses the Train Sequence Code in the TSC set 1, and the user over the sub-channel 2 uses the Train Sequence Code in the TSC set 2. At the reception side of the base station, the received signal for the two users is demodulated and/or deciphered using multi-user detection or interference cancellation techniques, and also a corresponding wireless link control process is perform for the two users.
In the VAMOS system, a typical user configuration scenario lies in that two full-rate TCH service channels are multiplexed over the same slot and frequency. As shown in FIG. 2, the voice service of the GSM system also includes a half-rate service, where a full-rate service occupies slots equal in quantity to those occupied by two half-rate services. Therefore, after the VAMOS multiplexing is adopted, the system allows two full-rate TCH users or four half-rate TCH users in maximum being multiplexed over the same physical resource, i.e. sharing the same slot number, frame number and ARFCN in the uplink and the downlink. The full-rate TCH user namely refers to the user using the full-rate TCH, and the half-rate TCH user namely refers to the user using the half-rate TCH. This is always the case hereinafter if no exception is clearly indicated in the context.
Therefore, a scenario occurs where “three users” are multiplexed together with each other: a full-rate TCH user+two half-rate TCH users, or alternatively, three half-rate TCH users (including: 1 half-rate TCH user+2 half-rate TCH users; 2 half-rate TCH users+1 half-rate TCH user, where the left side to the plus symbol refers to the TCH type over the VAMOS sub-channel 1 and the right side to the plus symbol refers to the TCH type over the VAMOS sub-channel 2). When any user in the user group has switched to an adjacent cell or terminates communication, the user group enters a “two-user” multiplexing situation, the specific scenarios of which include: a full-rate TCH user+a half-rate TCH user, a half-rate TCH user+a half-rate TCH user.
FIG. 4 is a structural diagram illustrating channels such as a TCH, a FACCH and an SACCH after VAMOS multiplexing is adopted (only one slot is considered in each frame). It should be noted that, the FACCH employs “stealing frame”, i.e. delivery is performed by occupying the TCH when the FACCH is required.
In FIG. 4(a), two full-rate traffic users are multiplexed together with each other to constitute a VAMOS group, and share the same frame number, slot number and ARFCN.
In FIG. 4(b), a full-rate traffic user and two half-rate traffic users are multiplexed together with each other to constitute a VAMOS group. The full-rate traffic user occupies a VAMOS sub-channel, and the two half-rate traffic users are time division multiplexed over another VAMOS sub-channel.
In FIGS. 4(c) and 4(d), three half-rate traffic users are multiplexed together with each other. Two half-rate traffic users are time division multiplexed over a VAMOS sub-channel, and the remaining half-rate service occupies another VAMOS sub-channel.
In FIG. 4(e), four half-rate traffic users are multiplexed together with each other. Specifically, two half-rate traffic users are multiplexed over a VAMOS sub-channel, and the remaining two half-rate traffic users are multiplexed over another VAMOS sub-channel.
As can be seen from FIG. 4, the VAMOS achieves the object of expanding the system capacity by multiple users multiplexing the same time-frequency resource, but also causes interference between the coupled users, which results in a decrease in system performance. Although the orthogonality between the multiplexed two users are ensured at the sending side, the existing three VAMOS candidate schemes(Co-TCH, OSC and alpha-QPSK) may lead to signal leakage between the two sub-channels at the reception side due to inter-symbol interference resulted from multi-path propagation characteristics of the wireless channel as well as non-linear characteristics of a sending filter and a reception filter. In the downlink, this means that the multiplexed two users have interference on each other. While in the uplink, orthogonal characteristics cannot be ensured due to a random phase difference between the users, even if no time dispersion fading occurs in the channel. Since the orthogonality between the two sub-channels cannot be ensured at the reception side, the multiplexed two sub-channels have interference on each other, which leads to occurrence of an intra-cell interference. In this case, in both the full-rate TCH and the half-rate TCH, the interference between each other results in occurrence of serious code errors in the SACCH frame since the SACCH frames of the coupled users are always sent simultaneously, which results in a significant decrease in the system performance.
In the GSM system, the SACCH is a very important channel. Messages for the GSM system are sent in two logic channels, i.e. the BCCH and the SACCH. In idle mode, the system sends the system messages 1˜4, 7 and 8 via the BCCH channel. In active mode, it delivers the system messages 5, 5bis, 5ter and 6 via the SACCH. Additionally, the downlink SACCH is also used to deliver the Layer-1 header messages, which include: communication quality, an LAI number, a cell ID, a BCCH frequency point signal intensity of the adjacent cell, a limit for the NCC, cell selection, a Timing Advance(TA) value and a power control level. The uplink SACCH bears a cell measurement report and a Layer-1 header message, which include: a signal intensity regarding the serving cell and the adjacent cell that is received by the mobile station, which is necessary for the mobile station to participate in switching, the TA value and the power control level. In addition, an MAC-layer massage is also delivered in the SACCH.
The SACCH contains 184 information bits in total, which are coded as 456 bits. An interleaving depth is 4. 456 bits are divided into 4 sub-blocks after being interleaved, and are mapped into 4 bursts. As shown in FIG. 2, for both the half-rate TCH and the full-rate TCH, the SACCH sub-block occurs in the 26-multiframe for only once. Therefore, a piece of complete SACCH information is composed of the SACCH sub-block in 4 continuous 26-multiframes. 4 26-multiframes are namely 104 TDMA frames, and therefore a period for the SACCH is 480 ms. In order to facilitate description, the 4 continuous SACCH sub-blocks are numbered as 0, 1, 2, and 3, and the 4 continuous 26-multiframes are called the SACCH frames which numbered as 0, 1, 2, and 3.
In view of importance of the SACCH and the fact that SACCH performance decreases in the VAMOS system due to the interference between the paired users, there is an urgent need to propose a new solution for enhancing the SACCH performance.
There is an existing SACCH enhancing method called “shifted SACCH”, which is used in the situation that four half-rate TCHs are multiplexed together with each other. The core idea of such method is to change the position of the SACCH frame of the two half-rate TCHs on the VAMOS sub-channel 2, so as to be separated in time from the SACCH frame of the two half-rate TCHs on the VAMOS sub-channel 2, thus avoiding two SACCH frames occurring simultaneously. Thus, the SACCH frame of each user respectively occurs simultaneously with the TCH frame of the coupled user over another VAMOS sub-channel, and when one of the users is in a DTX state and does not need to send any information, the interference on the SACCH frame of his/her paired user may be decreased, thus achieving the object of enhancing the SACCH performance. However, the SACCH frame separated in time may have an influence on the interleaving process for the voice frame, which leads to time sequence confusion. On the other hand, considering that the FACCH channel employs the “stealing frame” manner and only occurs along with the TCH, it is possible that the shifted SACCH may occur simultaneously with the SACCH sent over another VAMOS sub-channel.